Adding content
The Super Advent Calendar plugin lets you add content to the front and back of a day by selecting a particular day and opening the block panel settings.

Select a presentation style
You can choose to show the content directly on the card, in a modal or open a link to another website or internal page.
Using shortcodes
You can use shortcodes inside the text field to add more functionality e.g. a form, quiz, image gallery and so on. Read more about using shortcodes.